Wheeled vehicles such as trucks, which are designed to carry heavy pay loads, have been relatively hard riding due to the use by many manufactures of relatively high-frequency-rate springs. While low-frequency-rate springs have ben used, there has tended to be roll and there have been some problems in maintaining alignment.
To get a smooth ride, the present invention relies on a pneumatic spring to support the vehicle load. However, in order to locate the axle relative to the air spring and to provide roll resistance during braking, the air spring is mounted on a metal leaf spring. By having the rear end of the leaf spring retained in alignment but not shackled directly to the vehicle frame and by providing the air bag with a sensitive height control device, the invention provides the needed low-frequency spring rate through an air spring and a leaf spring in parallel, with the air spring carrying most of the load, the percentage split between the springs remaining constant through the load range. Yet the needed alignment and roll brake resistance are provided.
The closest prior art known at this time are two U.S. Pat. Nos. 3,031,179 to Peirce and 4,099,741 to Sweet et al.
The Peirce U.S. Pat. No. 3,031,179 uses an air bag for a first stage in a two-stage suspension with a metal spring for the second stage. Peirce notes that when the suspension is lightly loaded, the air spring supports the vehicle weight, but when the suspension is heavily loaded, the metal spring is in contact with the rear bracket, and the metal spring helps to carry the load. This differs from the present invention, in that in the Peirce patent the air spring is placed in series with the leaf spring when the suspension is heavily loaded.
In Sweet et al U.S. Pat. No., 4,099,741, a vertical load is placed through the shackle into the frame at the rear of the metal spring, whereas in the present invention no such load is transmitted. It should also be noted that the Sweet et al patent does not really show an air suspension, because the air bag carries only a small proportion of the load, about 1/8 of the load. The air spring is used by Sweet to give better control over the height of the frame rather than to provide air suspension. Since the Sweet air bag is offset from the axle, however, the front portion of the air spring does carry some load, but the relative stiffness of the metal spring in comparison with the low rate of the air bag means that the spring of the forward section serves more as a link than as a spring.
In the past, air bags have been used principally as a means for increasing the capacity of a suspension, or as a first stage in a two stage suspension like that of Peirce just discussed, or as a suspension spring on a variety of different air suspension systems.